1. Field of the Invention
The present invention generally relates to the determination of additives in metal plating baths, and more specifically to a method and apparatus for determination of organic suppressor and accelerator additives in semiconductor copper electrolysis plating baths.
2. Background of the Invention
Traditionally, aluminum (Al) has been used as the material of choice for metalization in forming interconnect layers in the manufacture of semiconductor microelectronic integrated circuits. Al is commonly deposited on semiconductor structures by chemical vapor deposition (CVD), which allows for precise control and highly uniform deposition of the product metal-containing film.
Despite the prior ubiquity of Al as a metalization medium, performance demands associated with increasing signal speeds and decreasing feature geometries of microelectronics have exceeded the capabilities of Al metal. Copper (Cu) therefore is increasingly being utilized as a semiconductor interconnect metal. The properties of Cu are not amenable to conventional CVD metalization approaches, due in part to the lack of suitable copper source reagents, and in consequence Cu is typically deposited on the microelectronic device structure via electroplating.
Electroplating of copper, however, has various associated problems.
Generally, Cu is plated onto a substrate by electrolysis in an etch solution, which may for example comprise copper sulfate, sulfuric acid, and hydrochloric acid. The plating process with an unaugmented etch solution of such type normally proceeds too rapidly. The result of such plating rapidity is that previously formed vias, i.e., passages to lower-level structures, e.g., electrodes or other conductors or semiconductor regions in the microelectronic device structure, are bridged over, and not filled with Cu. Accordingly, the desired electrical path to the underlying structure is not formed, and the semiconductor device structure must be reworked or discarded.
In order to combat such plating rapidity, the Cu plating process must be retarded. Additionally, the copper plating process requires acceleration in some aspects, to achieve desired coverage and leveling properties of the deposited metal. To achieve these concurrent opposing goals, organic additives are introduced into the copper electroplating bath to both slow down the plating process (suppressor additives) and to speed it up (accelerator additives). The speed of deposition of Cu on the substrate, and the quality and resulting electrical and mechanical properties of the metalization, are critically dependant on the concentration of these organic additives in the copper electroplating bath. However, the concentration of these additives is not constant, due to either xe2x80x9cdrag-outxe2x80x9d by the wafers or by electrochemical reaction and loss during the electroplating. Accurate, real-time measurement of these electroplating bath additive concentrations, necessary for quality control, has been problematic.
The respective suppressor and accelerator organic compounds in the copper electroplating bath are usually present at very low, e.g., part-per-million by volume (ppmv) concentrations. This circumstance makes normal analytical procedures difficult to effectively apply, due to the masking effect of the high concentration of inorganic bath components (copper, acid, etc.). The most effective way of determining these organic compounds is by measuring their effect on the amount of Cu deposited.
Methods of measuring the effect of the concentration of the electroplating suppressors and accelerators are known in the art. U.S. Pat. No. 5,192,403, issued to Chang et al. on Mar. 9, 1993, describes one such method, comprising the steps of:
a) preparing a basis solution which contains all of the components of the plating solution to be measured (the xe2x80x9csamplexe2x80x9d), except the component of interest;
b) preparing a calibration solution which contains the component of interest in a known concentration near that which would be expected in the sample;
c) adding measured amounts of the calibration solution to a first defined volume of the basis solution, and plotting the copper plating (cathodic) charge in cyclic voltammetry in the mixed solution against the added volume of the calibration solution;
d) adding measured amounts of the sample solution to a second volume of the basis solution, and plotting the copper plating (cathodic) charge in cyclic voltammetry in the mixed solution against the added volume of the sample; and
e) comparing the slopes of the calibration standard curve and the sample mixture curve to determine the concentration of the component of interest in the sample solution.
Variations of this technique are employed in the art to measure the concentrations of organic suppressor and accelerator additives in Cu electroplating baths for semiconductor manufacturing. These techniques variously measure the plating charge or stripping (de-plating) charge, e.g., for electro-plate deposition of Cu directly onto a test electrode via current supplied to a counting electrode in a plating step, and removal of previously plated copper in a stripping step. The charge is generally obtained by measuring the plating or stripping current while holding the voltage constant, and integrating to obtain the charge. Typically, an electrode is cyclically plated and de-plated (stripped of the previously deposited Cu) multiple times for each quantity measured. Each plating/measurement cycle comprises the following steps:
Cleanxe2x80x94the test electrode surface is thoroughly cleaned electrochemically or chemically using acid bath, followed by flushing with water or acid bath,
Equilibrate (optional)xe2x80x94the test electrode and a reference electrode are exposed to the plating electrolyte and allowed to reach an equilibrium state.
Platexe2x80x94Cu is electroplated onto the test electrode either at constant potential or during a potential sweep and the current between the test and counter electrodes is monitored and recorded, and
Stripxe2x80x94the Cu deposition is removed (e.g., by reversal of the plating current flow and/or exposure to an acid bath) by suitably changing the potential between the test and counter electrodes stepwise or in a sweep in the reverse direction and the current between the test and the counter electrode is monitored and recorded (and integrated to determine the xe2x80x9cstripping chargexe2x80x9d).
These four steps must be repeated for each plating/measurement cycle; each sample measurement is typically repeated several times to eliminate random errors introduced by variations in process conditions, e.g., composition, temperature, etc. Hence, an entire concentration determination sequence can require a considerable period of time to complete. To be useful as a quality control tool in copper metalization in semiconductor manufacturing, the concentration determination must be completed in a very short time frame so that significant depletion of the organic additives in the plating bath does not occur. Any significant depletion of organic additives during the determination will render the analytical method useless.
It would therefore be a significant advance in the art, and is accordingly an object of the present invention, to significantly reduce the time required for the concentration determination sequence to be completed, relative to the present state of the art.
To allow for fine control of the plating process, it is also desirable that concentration of organic additives be determined to a high degree of accuracy. Is therefore is a further objective of the present invention to determine the organic additive concentrations to a high degree of precision, preferably less than 10 percent of indicated value, and more preferably less than about five percent of indicated value.
It is another object of the invention to provide an improved system for determination of organic additive concentration in a copper electroplating bath, that is simple in operation, economic in capital cost and operating expense, and efficient in characterization of the electroplating medium.
Other objects and advantages will be more fully apparent from the ensuing disclosure and appended claims.
The present invention relates in one aspect to an apparatus for the determination of concentrations of organic additives in a Cu electroplating bath, comprising
a reference electrode, housed in an electrically isolated reference chamber and immersed in a base metal plating solution;
a test electrode having a plating surface upon which metal is depositable by electroplating, disposed in a measurement chamber containing an electroplating current source electrode, wherein metal plating solutions containing unknown concentrations of additives are introduced to, and intermixed with, the base metal plating solution;
a capillary tube joining the reference chamber and the mixing chamber in unidirectional fluid flow relationship, whereby base metal plating solution is transferred to the measurement chamber from the reference chamber, and wherein the measurement chamber end of the capillary tube is disposed in close spatial relationship to the plating surface of the test electrode;
selectively controllable electroplate driving electronics electrically and operatively coupled between the test electrode and the electroplating current source electrode, whereby metal is selectively deposited onto the test electrode from the mixed metal plating solution in the mixing chamber at a constant or known current density; and
electrical potential measuring circuitry electrically and operatively coupled between the test electrode and the reference electrode, whereby electrical potential between the electrodes is measured and recorded.
The present invention relates in another aspect to a method for measuring the characteristic decisive potential of a mixed metal plating solution by performing a plating/measuring cycle, comprising:
cleaning the test electrode and measuring chamber by a method selected from the group consisting of acid bath exposure, electrolytic cleaning with or without gas (oxygen) generation and water flush, and combinations thereof;
flowing a first known volume of base metal plating solution without the component of interest from the reference chamber through the capillary tube into the measurement chamber;
optionally adding to the measuring chamber a second known volume of metal plating solution containing some concentration of the component of interest and mixing the solutions;
allowing the test electrode to come to an equilibrium state in the mixed metal plating solution, such that there exists no electrical potential between the reference electrode and the test electrode;
depositing metal onto the test electrode from the mixed metal plating solution in the mixing chamber by electroplating at a constant or known current density;
measuring and recording the decisive electrical potential between the reference electrode and the test electrode at a set time after initiation of the plating step, whereby sufficient stability has been reached;
measuring and recording the equilibrium electrical potential between the reference electrode and the test electrode following completion of the plating step, whereby the current flow in the electroplating circuit is zero;
calculating the over-potential by subtracting the equilibrium potential from the decisive potential;
stripping the deposited metal from the test electrode by a method selected from the group consisting of chemical stripping, application of reverse bias electroplating current, and combinations thereof.
The present invention relates in another aspect to a method for conditioning the base plating solution for the determination of organic additives in metal plating solutions, comprising:
adding to the first known volume of base metal plating solution in the measuring vessel a known volume of additive and performing plating and stripping operations, whereby the non-linearity of the response of the decisive potential to the additive is xe2x80x9cmasked,xe2x80x9d and all decisive potential measurements are carried out in the linear region of the response, this optional conditioning of the base metal plating solution being performed prior to the introduction of the sample to be determined.
The present invention relates in another aspect to an algorithm for determination of the concentration of organic additives in metal plating solutions, comprising:
plotting values calculated as the inverse of the ratio of the measured potential of each metal plating bath solution containing additives to the measured potential of the metal plating bath solution containing the sample, minus one;
linearly extrapolating back through these points to determine the point corresponding to the value of the inverse of the expression:
[(the measured potential of metal plating for that solution, with no additives)/(the measured potential of metal plating for that solution, containing the sample)]xe2x88x921; and 
calculating the negative inverse of the value.
The present invention is based in part on applicant""s discovery of a technique to dramatically reduce equilibration time of the reference electrode in an apparatus for the determination of concentration of additives by the Pulsed Cyclic Galvanostatic Analysis (PCGA) technique. In conventional practice, a reference electrode is disposed in the same electrolyte solution as is the test electrode upon which Cu is deposited. Following each plating/measurement cycle, the test electrode must be stripped of the deposited Cu, and cleaned to remove all traces of the test solution (which contains some level of additive). The test electrode and the reference electrode are then re-immersed in the base copper plating electrolyte solution, and must return to an equilibrium state prior to initiation of the next plating/measurement operation.
In one embodiment of the present invention, the reference electrode resides in a reference chamber that is physically and electrically isolated from the mixing chamber that houses the test electrode (upon which Cu is deposited). The reference electrode is continuously immersed in the base copper plating electrolyte solution. By never exposing it to the variously doped bath solutions in the mixing chamber, the reference electrode need not be cleaned following each plating/measurement cycle. Thus, it remains continuously xe2x80x9cequilibratedxe2x80x9d to the base copper plating electrolyte solution, and the equilibration step is reduced to the time necessary for the test electrode to xe2x80x9cequilibratexe2x80x9d to a fresh base copper plating electrolyte solution. This reduces the equilibration step by roughly an order of magnitude over the prior art, i.e., to approximately two minutes.
The reference chamber is connected in fluid flow relationship to the mixing chamber by a capillary tube, whose mixing chamber terminal end is in close physical proximity to the plating surface of the test electrode. By this arrangement, the apparatus of the present invention achieves several additional advantages, including:
Potential difference (iR drop) across the electrolyte is eliminated or dramatically reduced.
The measuring chamber is filled with base copper plating electrolyte solution for each cycle through the capillary tube, from the reference chamber. Both electrodes are hence initially immersed in the same electrolyte.
The flow of base copper plating electrolyte solution through the capillary tube and against the plating surface of the test electrode facilitates the removal of air on the test electrode, contributing to consistent cycle-to-cycle measurements.
The flow of base copper plating electrolyte solution through the capillary tube generates a fresh and reproducible liquid junction to the measuring vessel.
Additional aspects, features and embodiments of the invention will be more fully apparent from the ensuing disclosure and appended claims.